The present invention relates to a cable, in particular a cable for electric power transmission or distribution or for telecommunications. In more detail, the present invention relates to a cable as above defined comprising at least one outer coating sheath and provided with self-repairing protection which is capable of restoring the continuity of the coating sheath after it has been broken.
Electrical cables, in particular low- or medium-voltage cables for the distribution of electric energy for domestic or industrial use, generally consist of one or more conductors individually insulated by a polymeric material and coated with a protective sheath, which is also made of a polymeric material. These cables, in particular when installed underground, either directly or inserted in tunnels or inside buried pipes, are subjected to damages on these layers caused by various types of mechanical abuses, for example accidental impact with sharp tools such as shovels or picks, which exert both cutting and compression actions on the cable, This can lead to partial or complete rupture of the outer sheath and possibly also of the inner insulating layer, which will bring about infiltration of moisture and generation of leakage currents. If rupture of the coating layers reaches the conductor, the combined effect of leakage currents and moisture leads to a gradual corrosion of the conductor until, at the most, a complete breakage of the conductor itself.
To obtain effective protection against such mechanical abuses, the cable can be provided with an outer structure capable of withstanding both cutting and compression, this outer structure consisting of a sheath made of a metal or a plastic material combined with a metal armouring, for example. In addition to being expensive, this solution leads to an important increase in the overall dimensions and rigidity of the cable, thus making this solution unsuitable for cables requiring easy installation and low costs, such as, in particular, in the case of low-voltage cables.
In Patent Application DE-1,590,958 a telecommunications or high-current cable is described which is protected from mechanical damages by means of an outer sheath provided, on its inside, with microcapsules containing a liquid that is capable of rapidly solidifying, once the microcapsule has been broken. To this purpose, use of the two components commonly employed for manufacturing expanded polyurethane is mentioned as the preferred one, these components being microencapsulated separately so that they react together on breaking of the microcapsules, forming an expanded material which closes the accidental cut. Alternatively, liquids solidifying when brought into contact with external agents, moisture for example, may be used.
According to the Applicant, the solution envisaged in the above-mentioned patent application is of difficult practical implementation and has many drawbacks. Firstly it is to note that the possibility of self-repairing is limited to the outer sheath, and no indications regarding the possibility of restoring integrity of the inner insulating layer are provided. In addition, to obtain an effective self-repairing effect, it is necessary to introduce a large amount of microencapsulated material during sheath extrusion, which operation can be rather difficult and also expensive. It is finally to be pointed out that the mechanism of action of the microcapsules is irreversible, so that the self-repairing effect can be carried out only once, i.e. at the moment the microcapsules are broken. Actually, during the various stages of the cable life (manufacturing, storage, installation, use), the coating layers are inevitably subjected to external mechanical actions of compression and bending and to thermal cycles of expansion and compression, which can lead to rupture of the microcapsules with consequent expansion and/or solidification of the material contained therein. This material therefore, will be no longer able to effect the desired self-repairing action when the sheath is actually damaged. It is also to be noted that, even when microcapsules are used which contain a liquid material solidifying on contact with moisture, accidental rupture of the microcapsules without any actual damage to the outer sheath leads in any event to solidification of the material because inside the cable there is always some residual moisture.
The Applicant has now found that, in consequence of a mechanical damage creating a discontinuity in at least one of the cable coating layers, it is possible to obtain effective self-repairing of the coating by virtue of the presence of an inner layer, placed, for example, between the insulating layer and the outer sheath, and comprising a material having a predetermined cohesiveness and at the same time a controlled flowability, which is capable of repairing the damage by restoring the continuity of the coating layer. After a discontinuity in the coating has been created, the material xe2x80x9cmovesxe2x80x9d towards the damaged point and fills up the discontinuity at least partly by forming a substantially continuous layer which is capable of maintaining the cable functionality under the expected working conditions.
The action of the self-repairing material taking place with a reversible mechanism, among other things, prevents moisture infiltration and establishment of leakage currents, and consequently quick corrosion of the conductor.
Based on this starting perception, the Applicant has developed and set up a self-repairing cable and related manufacturing process, being the object of the Patent Application EP 99103092.5, contents of which is considered as herein reported for supplement and completion of the detailed description of the present invention as hereinafter set forth. In accordance with the present invention, the Applicant has now found that by arranging one or more anchoring portions between the outer sheath and the core of the cable, each housed in an interruption region of the self-repairing material extension, further improvements can be advantageously achieved in terms of cable reliability. In particular, any possibility of relative sliding between the outer sheath and inner core of the cable is advantageously eliminated, independently of whether said core is made up of one or more bare conductors or of conductors provided with one or more coating layers internal to the sheath.
In addition, also solved are problems resulting from unsteady positioning of the conductor within the self-repairing material bringing about off-setting of the conductor relative to the cable axis and thickness unevenness in the self-repairing layer itself.
More particularly, the present invention relates to an electrical cable with self-repairing protection comprising: at least one conductor; at least one outer coating sheath; characterized in that it further comprises: at least one layer of self-repairing material interposed between the conductor and the outer coating sheath, the self-repairing material layer being distributed around the conductor and having at least one region wherein its extension is interrupted; and at least one anchoring portion between the conductor and the outer coating sheath, disposed at said interruption region.
In particular, a plurality of anchoring portions homogeneously distributed around the conductor is preferably provided, each portion being placed at an interruption region of the extension of the layer of self-repairing material.
The layer of self-repairing material is conveniently provided to extend around the conductor following a distribution line along which the ratio between the extension of the self-repairing material layer and the extension of the interruption regions is at least equal to 0.5, and preferably included between 0.5 and 10, more preferably between 0.7 and 2.
The layer of self-repairing material and said at least one anchoring portion can be advantageously disposed directly in contact with the conductor.
In a preferred embodiment, it is however provided that at least one inner coating layer is interposed between the conductor and the layer of self-repairing material.
Each anchoring portion is conveniently directly put into contact with, and possibly joined in one piece to, the inner coating layer.
It is also preferably provided that the anchoring portion or portions should be put directly into contact with, and preferably joined in one piece to the outer coating sheath.
The Applicant has further found convenient for the self-repairing material layer to have a thickness not lower than 0.1 mm.
According to a further aspect, the present invention relates to a method of manufacturing an electrical cable comprising the step of externally applying an outer coating sheath around at least one conductor, characterized in that it further comprises the following steps: applying at least one layer of self-repairing material between the conductor itself and the outer coating sheath; forming at least one interruption region in the extension of said layer of self-repairing material; disposing at least one anchoring portion between the conductor and the outer coating sheath at said interruption region.
In particular, a plurality of said interruption regions homogenously distributed around the conductor is preferably formed and a plurality of anchoring portions are disposed each at one of said interruption regions.
According to a first embodiment of the present invention, the interruption region of the extension of the self-repairing material layer is formed by removing part of the applied self-repairing material from said conductor.
The self-repairing material and anchoring portions can be directly applied to the conductor.
Alternatively, at least one inner coating layer is applied to the conductor before carrying out application of the self-repairing material layer. In this case, the self-repairing material and the anchoring portions are applied directly in contact with the inner coating layer, and possibly accomplished simultaneously, using the same material forming said inner coating layer so as to define one single body on the conductor.
In addition, the anchoring portions are preferably put directly into contact with the outer coating sheath, and possibly manufactured simultaneously with said sheath, to define one single body circumscribing the conductor.
In accordance with a second embodiment of the method in accordance with the present invention, the anchoring portions, the outer coating sheath and the inner coating layer are made of the same coating material, so as to form a unitary body.
Preferably, application of the self-repairing material layer is carried out by injecting the material itself into said coating material, concurrently with the simultaneous accomplishment of the inner coating layer, the anchoring portions and the outer coating sheath.
The present invention also relates to an apparatus for manufacturing electrical cables with self-repairing protection, comprising at least one guide head having at least one inlet opening and at least one outlet opening through which at least one conductor is lengthwise moved; first application devices fed with a coating material and connected to said outlet opening for depositing at least one outer coating sheath around the conductor, characterized in that it further comprises: second application devices operatively associated with the guide head for depositing at least one layer of self-repairing material around the conductor, said second application devices being arranged to define at least one interruption region of the layer extension in the layer of self-repairing material.
In accordance with a first preferred embodiment, the second application devices comprise: at least one storage chamber for the self-repairing material located in the guide head between said inlet opening and outlet opening, said storage chamber and said self-repairing material being passed through by the conductor moving towards the outlet opening; at least one extrusion tip disposed at said outlet opening and arranged to remove at least part of the self-repairing material layer from the conductor to define said at least one interruption region.
In more detail, the extrusion tip preferably has one or more forming teeth homogeneously distributed around the conductor, which act in abutment relationship relative to the conductor to form said interruption region, each forming tooth having at least one conveying surface converging towards the conductor in the feeding direction of the latter so as to delimit, in the first application devices, at least one application channel so as to bring part of said coating material to said interruption region.
In a further preferred solution, the second application devices comprise at least one dispensing nozzle fed with the self-repairing material and operatively associated with said first application devices to inject the self-repairing material into the coating material flowing towards the outlet opening.